Fireproof acrylonitrile copolymers



United fitates Patent @fifice 3,027,222 Patented Mai". 27, 19623,027,222 FIREPR-QQF ACRYLGNHTRHLE CGPOLYMERS William KennethWiiirinson, Waynesboro, Va., assignor to E. ll. du Pout de Nemours andCompany, Wilmington, Dei., a corporation of Deiaware No Drawing. FiledSept. 3, 1957, Ser. No. 681,477 4 Claims. (Cl. 13-4155) This inventionrelates to fireproof organic fibers. More specifically, it relates tofireproof fibers made from copolymers of acrylonitrile.

It has been reported by Houtz (Textile Research Journal, 20, 797 [1950])that yarns prepared from acrylonitrile homopolymer undergo a changeduring heating at 200 C. which results in black color and fireproofproperties for the yarn. After 16 hours at this temperature, the yarnsreach a condition in which they will not burn when placed in the flameof a Bunsen burner.

These yarns are, as Houtz discloses, brittle by textile standards.Because of this excessive brittleness, the Houtz procedure isnecessarily limited to fabrics rather than yarns or individual fibers.Fibers heat-treated in this manner are too brittle to be spun into yarnsand yarns cannot be woven or knit into fabrics. Even the heattreatedfabrics themselves are sufficiently brittle to prevent their acceptancein many applications requiring fireproof fabrics, in spite of theexcellent fireproof qualities they possess.

It is an object of this invention to provide fireproof fibers that areprocessable on textile equipment. It is a further object to providefireproof fibers of good abrasion resistance. It is yet another objectto provide an economical process for the preparation of fireproof fibershaving good textile properties. Other objects and means foraccomplishing them will appear hereinafter.

In accordance with this invention there are prepared from a syntheticorganic polymeric composition fireproof fibers which are sufiicientlystrong and flexible to be spun into textile yarns. The polymericcomposition which is formed into fibers preparatory to producing thestrong flexible fireproof product contains between about 85% and 99%acrylonitrile in polymer form and at least 1% of another monomer also inpolymer form. It is important that the polymeric composition contain nohalogens attached to any polymer chain. The strong flexible fireprooffibers of this invention are prepared by heating fibers of such apolymeric composition from a temperature lower than 20 below the sticktemperature of the fibers to a temperature above about 260 C., theheating being controlled so that the temperature of the fibers rises ata rate slow enough to prevent the fibers from sticking to each other.Upon reaching a temperature above a temperature of about 260 C., heatingmay be continued at the same controlled rate or may be discontinued solong as the temperature of the fibers remains above about 260 C. untilthe fibers become fireproof.

The total time of heat-treatment required to render the fibers of thisinvention fireproof will usually range from about 1 to about 3 hours,depending upon the initial and maximum temperature employed. The sticktemperature of the fibers must be at least l80 C. if the advantages ofthe invention are to be obtained, and preferably the stick temperaturewill be between about 230 C. and about 260 C. It is critically importantalso that the fibers undergoing heat-treatment be in the form of afibrous mass having a maximum bulk density of less than about 0.2 gramper cubic centimeter. For the most uniform results, the density of thefibrous mass should be substantially uniform throughout or, at the veryleast, no por tion of the fibrous mass should have a density greaterthan 0.2 gm./ cc.

In carrying out the heat-treatment procedure of this invention, thefibers are placed in a heating chamber in such a manner that the bulkdensity of the fibrous mass is less than about 0.2 gm./cc. Best resultsin terms of flexibility and strength of the product are obtained if thisdensity is less than about 0.1 gm./cc. The fibers may be present in theform of card webs, staple batts, needle punched batts, low twist yarns,or loosely woven or loosely knit fabrics, provided that the bulk densityof the material to be heated does not exceed 0.2 gm./cc.

The process of this invention is preferably applied to fibers arrangedin the form of a loose batt in which fiber orientation is random, thebatt being less than 10 inches in depth. Continuous filaments as well asstaple fibers may be processed, but the latter are preferred as moresuitable for preparing loose uniform batts of a substantially uniformdensity below 0.2 gm./ cc. When the process is applied to continuousfilaments, yarns, or fabrics, care must be taken so that adjacentfilaments are not so closely associated as to create a zone having abulk density exceeding 0.2 gm./ cc. Yarns must be very loosely spun sothat the maximum density of the yarn bundle does not exceed the abovedensity limit. Fiber bundles such as, for example, tow are much easierto convert into strong flexible fireproof fibers according to thisinvention than are yarns or fabrics. Theprocessing of fibrous massescontaining zones with bulk densities exceeding the density maximum setforth above results in fireproof products which are non-uniform withrespect to strength and flexibility, the zones with excessive densitybeing too brittle and fragile for textile applications. The ultimateutility of such a fibrous mass will depend, of course, upon the expanseof these zones and the extent to which they dominate the strengthcharacteristics of the fibrous product.

Fibers useful as starting materials in the process of this inventionhave fiber stick temperatures above 180 C. and have been oriented duringtheir production by drawing to at least 300% of their original as-spunlength. These fibers are formed from acrylonitrile polymers and polymermixtures in which the combined acrylonitrile content is between and 99%by weight. With less than 85% acrylonitrile, a fireproof structure doesnot necessarily form. With more than 99% of acrylonitrile, the fireproofproduct obtained is too brittle to be of practical value. Of particularvalue are those copolymers of acrylonitrile containing between 5 and 15%of other vinyl monomers, mixtures of copolymers of acrylonitrile inwhich the total amount of other vinyl monomers is between 5 and 15%.

Copolymers and terpolymers of acrylonitrile are preferred polymericcompositions for preparing the fibers which are heat-treated in thisinvention, but other acrylonitrile polymer compositions may also beused. In general, any polymer of acrylonitrile containing between about85% and 99% acrylonitrile is useful so long as there are no halogensattached to the polymer chain and the fiber formed from the polymer hasa stick temperature of at least C. Exemplary monomers which may bepolymerized with acrylonitrile to prepare polymers useful in thisprocess include monoethylenically unsaturated monomers such as acrylicacid, methacrylic acid, and the corresponding amides and alkyl esters ofthese acids, styrene, sodium styrene sulfonate, vinyl pyridine andsubstituted vinyl pyridines, vinyl acetate, N- vinyl pyrrolidone, vinylsulfonic acid, vinyl carbazole, N-phenyl maleimide, itaconic acid andits non-ha1ogenated derivatives, and the like. Other suitable monomersare disclosed in U.S. 2,436,926 and US. 2,743,994.

Fibers which may be successfully heat-treated by the process of thisinvention may also be prepared from mixtures of polymers, which mixturescontain between about 85 and 99% acrylonitrile in polymer form andbetween 1% and about 15% of another monomer copolymerizable withacrylonitrile and also in polymer form. Thus, for example, mixtures ofacrylonitrile homopolymer and poly(N-vinyl pyrrolidone) in the ratio offrom about 85/15 to 99/1, or a 90/10 mixture of an acrylonitrilecopolymer and acrylonitrile homopolymer, said copolymer containing 85%combined acrylonitrile and 15% combined methyl acrylate, may besuccessfully employed when in the form of fibers having a sticktemperature of at least 180 C. Similarly, a mixture of a terpolymer ofacrylonitrile with acrylonitrile homopolymer or with a polymer of anon-halogenated monoethylenically unsaturated polymerizable monomer or amixture of an acrylonitrile terpolymer with an acrylonitrile copolymeror a mixture of acrylonitrile copolymers may be utilized so long as thetotal combined (in the form of polymer) acrylonitrile present in thepolymeric mixture is between about 85% and 99%, the remainder beinganother monomer (in polymer form) copolymerizable with acrylonitrile.Obviously, it is necessary, utilizing a mixture of acrylonitrilepolymers, to employ polymers which are compatible in order that usefulfibers may be prepared therefrom. In any case, it is essential that thefibers utilized have a stick temperature of at least 180 C. and are freefrom polymers containing halogens attached to the polymer chain.

In carrying out this invention, the fibers are placed in a heatingchamber at a temperature about 20 C. or more below the fiber sticktemperature of the fiber to be treated. The temperature of the fibers isthen raised gradually until a temperature of about 260 C. is exceeded,taking care to control the rate of temperature rise so that the fibersdo not become overheated and stick to each other. Raising the fibertemperature at a rate of between about 0.3 C. and about 3 C. per minuteis satisfactory, and a rate of between about 1 C. and about 2 C. perminute is preferred as providing the most desirable products in theshortest reaction time. Using a rate of from about 1 C. and about 3 C.per minute, temperatures as high as 500 maybe reached without destroyingthe fibers, but temperatures of 300 are suificient to fireproof any ofthe fibers in practical times and optimum product properties areobtained with temperatures not exceeding 280 C.

In accordance with a preferred embodiment of this invention, a fiberhaving a stick temperature between about 240 C. and 260 C. is heatedfrom 220 C. to 275 C. at the uniform rate of about 1 C. per minute andmaintained at 275 C. for at least 5 minutes. The product has excellentstrength and flexibility properties and is fireproof.

The stick temperature of a fiber is measured by heating a copper blockto a selected temperature and holding the fiber against the surface ofthe block for five minutes under a pressure of 18 grams per squarecentimeter. By successive tests at different block temperatures, theminimum block temperature at which the fiber does not adhere to theblock at the end of the fiveminute period is determined and isdesignated as fiber stick temperature.

The term fireproof is defined as the total resistance to destruction ofa material by an open flame. A fireproof fiber does not burn even whilein contact with an open flame although it may glow.

The preferred heating medium is air. Other gas mixtures and variousliquid-heating media which have an oxidizing effect may also be used butair gives completely acceptable results and is generally chosen forreasons of practicality. The apparatus used for heating is preferablychosen to give a continuous fresh supply of hot air.

The products of my invention are useful wherever fireproof fibers aredesired. For example, they may be used in protective clothing to be wornby fire fighters or for those manipulating hot objects. Such protectiveclothing may most readily be made from non-woven felts prepared asdescribed in the examples. The bulk of these felts gives themparticularly good insulating power as compared to typical woven or knitstructures. In particular, gloves, suits, boot liners, and helmet linersmay be made and used. Such felt fabrics have many non-apparel uses suchas in hot pads for kitchen use and in insulating blankets.

The material is also useful in operations where the ultimate need is notresistance to temperature but where a high temperature step is part ofthe process. For example, gaskets may be cut from the fireproof felt andimpregnated with dispersions of tetrafluoroethylene polymer. Theimpregnated gaskets may then be heated to sufficient temperature to fusethe polytetrafluoroethylene into a continuous mass so that afiberreinforced polytetrafluoroethylene gasket will result. This gasketwill have extreme chemical resistance and will be in the desired formbecause of the resistance of the fireproof fiber to changes in physicalcondition during. the heating necessary to fuse the polymer.

For fireproof uses, the fibers are useful in other forms in addition tothe above-mentioned felts. Loose fibers may be used as insulation. Thefibers may be converted to shun yarns and thence to woven or knitfabrics usmg the usual textile procedures. These fabrics, like thefelts, will be completely fireproof. The fibers may also be blended withother fibers to produce yarns and fabrics that are not completelyfireproof but are considerably more fiarne resistant than the fibersused in the blending. For example, a yarn prepared from acrylonitrrlefiber and 50% of the fireproof fiber from an acrylonitrile fiber showsconsiderably reduced burning time over yarn from the acrylonitrile fiberalone and also shows none of the melting and dripping that is found forthis material.

The fireproof fibers of this invention are particularly useful in thepreparation of filters to be used for separation of solid impurities inhot gases. Bags sewn from woven fabric are most applicable for this usebut in many instances the fiber may be used in other forms.

The invention will be better understood from the following examples inwhich parts are by weight unless otherwise specified. All temperaturesare in degrees centigrade. The Stoll flex test is ASTM Test No. D1175T(1955 ed., Part 7, p.179).

EXAMPLE I A copolymer containing 94 parts of combined acrylonitrite and6 parts of combined methyl acrylate is dissolved in dimethylformamideand converted to staple yarn by a conventional process of dry spinning,solvent extraction, drawing, crimping, and cutting. These fibers have astick temperature of 249 C.

The staple fibers thus prepared, having a denier of 3.0 and a length of2 /2 inches, are spread loosely on a tray in a forced-draft oven to forma loose batt 5.0 centimeters thick covering an area of 225 squarecentimeters. The layer weighs 15 grams and has a uniform density of0.013 gram per cubic centimeter (gm/co). The temperature of the oven is220 C. The temperature of the fibers is raised gradually from 220 C. to275 C. over a period of minutes and maintained at the latter temperaturefor 5 minutes before removal. The product is still in the form ofindividual staple fibers which do not stick to each other and uniformlyblack in color. After application of Avitex R finish (a /2% aqueoussolution of a long chain fatty acid amine (C C the staple is carded on aconventional mechanical carding machine with no more fiber loss byfibrillation than is experienced with unheat-treated fiber of the samecom position. When the heat-treated fibers are held in the flame of aBunsen burner, they glow but do not burn. There is no after-burning orafterglow when the staple is removed from the flame and the filamentsstill do not stick to each other.

5 EXAMPLE II Staple fibers of 3.0 denier per filament and 2.5-inchlength are prepared as in Example I but using a terpolymer of 94%acrylonitrile, 5.7% methyl acrylate, and 0.3% sodium styrenesulfonate.The fiber stick temperature is 241 C. A portion of this staple isarranged in a forced-draft oven at 220 C. in such a way that 20 grams offiber have a volume of 500 cc. and a substantially uniform density of0.04 gm./cc. The temperature of the fibers is raised uniformly andgradually from 220 C. to 275 C. over a period of 60 minutes (less thanl/min.) and the latter fibers temperature maintained for 10 minutesbefore removing the fibers. The product, when removed from the oven, isuniformly jet black in appearance and is composed of individual fiberswhich do not adhere to each other. These fibers have an average tenacityof 1.02 grams per denier and an average elongation of 13.9% as measuredat room temperature and 65% relative humidity. When tested with a Bunsenburner as above, the staple is found to be fireproof.

A portion of the fireproof staple fibers prepared above is carded andthen spun into an 880-denier yarn. Samples of this yarn are weightedwith 45-gram Weights and are tested for abrasion resistance using anapparatus consisting of a reciprocating bar 3 inches above a horizontalAlsimag pin, /8 inch in diameter. One end of the yarn strand is attachedto the reciprocating bar and the strand is then caused to pass 360around the pin, thence 180 around the portion of itself leading from thereciprocating bar to the pin and thence 90 around the pin in thedirection whence it came, the free end hanging vertically and beingattached to a 45-gram Weight. The reciprocating motion of the bar causesthe yarn to rub on the Alsimag pin and on itself. This motion iscontinued for a sutficient number of cycles to cut the yarn completelyin two. The fireproof yarns Withstand an average of 130 cycles beforebreaking. They Withstand 23 cycles of a Stoll flex test.

For comparison, a number of fabrics prepared from polyacrylonitrile aremade fireproof by heating at 255 C. for 4 hours. The fabrics includesatins, twills, and basket weaves. Yarns removed from these fabrics havedeniers varying from 574 and 612. Abrasion resistance, as measured bythe above-described procedure, varies between 3 and 13 cycles. Thus, atenfold improvement in this important property is realized by the use ofthe process of the present invention. The control product fiberswithstand only 2 cycles of a Stoll flex test (30,000 denier per inch).

EXAMPLE III A quantity of 3-denier, 3-inch staple fiber prepared fromthe terpolymer of Example II is carded and several layers of the cardweb are placed on top of each other to form a batt about 1 /2 inchesthick with a specific gravity of about 0.003. This batt is passedthrough a needle loom to punch a number of the fibers into the batt inthe direction of its thickness, i.e., roughly perpendicular to the topand bottom surfaces. The needling action occurs about 50 times persquare inch of batt surface. After needle-punching from the top of thebatt, it is turned over and run through the needle loom again to punchit from the other side. The specific gravity of the batt is increased toabout 0.04 by the needling operation. The needle batt is placed in aforced-draft oven at 215 C. and the temperature of this oven is raisedgradually and uniformly to 275 C. over a period of 60 minutes and isthen held at this latter temperature for 5 minutes. The product removedfrom the oven is a black nonwoven fabric with pleasing aestheticproperties. Its surface area is approximately /2 that of the needledbatt placed in the oven originally. The needled fabric is pliable,strong, and tough, and can be sewn readily on a sewing machine to formfireproof garments. A glove is fabricated in which the seams are asclose as inch to the cut edges of the felt. This glove shows manyadvantages over asbestos gloves commonly used for handling hot objects.Because of the flexibility of the fabric, the glove is compliant withthe result that manual operations are much more readily and surelycarried out than when an asbestos glove is worn. The weight of the blackfelt glove is only /6 that of the asbestos glove and yet its insulativevalue is found to be twice as high. The felt, like the heated staple ofthe previous examples, is completely fireproof. Washing the glove in acommercial washing machine for 2 hours at 165 F. does not alter theproperties or appearance of the glove.

EXAMPLE IV Using staple fibers prepared from the terpolymer ofacrylonitrile, methyl acrylate, and sodium styrenesulfonate described inExample II, a series of tests is carried out to demonstrate thecloseness of packing of the fibers which may be used during the heattreatment in order to obtain a useful fireproof fiber. Samples oflooselycarded staple fibers, needle-punched batts, low-twist spun yarns,and loosely-knit fabrics are placed in a forceddraft oven and heatedgradually and uniformly from 220 C. to 275 C. over a period of 60minutes and held at this latter temperature for 5 minutes. Examinationof the products shows that those prepared from physical arrangements ofthe fibers having a uniform bulk density of less than 0.10 gram per cc.are practically indistinguishable from each other in terms ofbrittleness, all having excellent flexibility. The products from thesamples having a uniform bulk density between 0.11 and 0.20 gm./cc. showsomewhat more fibrillation during hand-carding after careful separationof individual filaments from their treated form. These samples are stillsufiiciently pliable, however, for the samples to be utilized in thepreparation of fireproof articles. The products from original samples ofbulk density about 0.20 are so brittle as to be of no practical valueand in some cases are not completely fireproof.

In another series of tests, samples of the same staple fibers are placedin a forced-draft oven in difierent bed thicknesses, the bulk densitybeing in all cases uniform from about 0.10 to about 0.20 gm./cc. It isfound that fireproof staple of good physical properties can be obtainedby heating the fibers at a slow enough rate to prevent sticking of thefibers, as above described, as long as the depth of the bed of fibersdoes not exceed about 10 inches.

EXAMPLE V Samples of needle-punched batts are prepared as in ExampleIII. One sample is placed in a forced-draft oven at 220 C. and raisedgradually and uniformly to 275 C. in 60 minutes and is held at thislatter temperature for 10 minutes. Other samples are placed in the ovenat 275 C. and held there for 20, 40, 60, and minutes, respectively. Yetanother sample is placed in the oven at 250 C., raised to 275 C. over 60minutes, and held at the latter temperature for 10 minutes. Only thesample placed in the oven at 220 C. is found to produce a pliable felt.The other samples show occasional fusing of filaments and thosefilaments which can hey removed from the felt show low elongation (about4 a Still other samples are placed in the oven at 220 C. and maintainedat that temperature for 10 and 20 hours. The sample removed after 10hours is found to be in completely fireproof. The sample removed after20 hours is fireproof but does not show a fabric strength comparable tothe felt prepared by heating gradually from 220 C. to 275 C. and thenheating 5 minutes at the latter temperature.

EXAMPLE VI Staple fibers are prepared from a number of polymers andpolymer mixtures containing acrylonitrile. Compositions of thesematerials are given in Table I which also gives the fiber sticktemperatures. Batts of each item are prepared using hand cards and areput through various heating cycles within the limits of this invention.The staple batts all have uniform bulk densities between 0.02 and 0.05gm./cc. All products are black and fireproof-except as indicated.

in Table 1, AN refers to acrylonitrile, MA is methyl acrylate, SSS issodium styrenesulfonate, MVP is 2-methyl-5-vinyl pyridine, VAc is vinylacetate, NVP'is N-vinyl pyrrolidone, VCI is vinylidine chloride, and VClis vinyl chloride, The designation AN/MA, 94/6 refers to a copolymercontaining 94% combined acrylonitrile and 6% combined methyl acrylate;AN/MA/SSS, 94/57/03 is a terpolymer containing 94% combinedacrylonitrile, 5.7% combined methyl acrylate, and 0.3% combined sodiumstyrenesulfonate; SOAN/50MVP//95AN/5VAc, 8/ /92 refers to a polymermixture containing 8% of a copolymer consisting of 50% combinedacrylonitrile and 50% combined 2- methyl-S-vinyl pyridine and 92% of acopolymer consisting of 95% combined acrylonitrile and combined vinylacetate. The other designations have corresponding connotations.

Table I Stick T empero- Produet Properties turc, 0.

Fiber Composition brittle.

flexible, tough.

moderately tough,

flexible, tough.

Do. Do.

Do. 295 moderately tough, ex c. 248 D0. D0. brittle, not fireproof.brittle, fused filarncnts. flexible, tough.

AN/itacouic acid, 97/53.. AN/NVP, 94/6 AN/VClz, 80/20 V AN/VCI, 40/60AN/styrene, 95/5 EXAMPLE VII taple fibers of the acrylonitrileterpolymer of Example II are heat-treated as shown in that example andcarded and spun into a 1200 denier yarn using conventional textilemachinery. Like all of the fiber products of this invention, yarns maybe spun from these fireproof flexible fibers in the conventional woolensystem. The spun yarn is then knitted on Wildman 23 inch knittingmachine, Model No. FBW (manufactured by Wildman Manufacturing Co.,Norristown, Pa.), at full speed (134 walcs/ 50 courses per minute) forthree minutes without a break showing that the yarn is eminentlysuitable for knitting.

EXAMPLE VIII The heat-treatment procedure of Example I is carried Cirout on staple fibers (3.0 denier, 1 /2 inches long) prepared from anacrylonitrile copolymer containing combined acrylonitrile and having astick temperature of 211 C. The fiber in the form of a loose batt havinga maximum bulk density of 0.05 is heated at the rate of 1 C./rnin. from190 C. up to a temperature of 300 C. and removed from the heating oven.The fibers do not adhere to one another and are black, fireproof, andsuficiently strong to be spun into yarns in the woolen system.

The above procedure is repeated except that a fibrous batt with a sticktemperature of 246 C. is heated at the rate of 3 per minute from 220 C.to 260 C. and maintained at 260 C. for 2.5 hours. The product fibers arestrong, flexible, and fireproof and suitable for further processing onconventional textile equipment.

The claimed invention:

1. In a process for providing fireproof properties by heating fibersspun from a linear polymeric composition essentially consisting ofbetween 85% and 99% acrylonitrile in polymer form and between 15% and 1%of other polymer copolymerized with acrylonitrile, the polymericcomposition containing no halogens attached to any polymer chain, thefibers having been drawn to at least 300% of their original as-spunlength; the improvement, for producing fireproof fibers which are strongand flexible enough to be spun into textile yarns, of heating saidfibers inthe 'form of a fibrous mass with a maximum bulk density of 0.2gram per cubic centimeter, having a fiber stick temperature of at least180 C., from a tem perature lower than 20 C. below the stick temperatureof the fibers at a rate of less than 3 C. per minute to a temperaturebetween about 260 C. and about 500 C., in an oxidizing gaseous mediumcontaining oxygen until the fibers become fireproof.

2. The process of claim 1 wherein the polymeric composition is acopolymer of acrylonitrile and a vinyl compound, and the fiber sticktemperature is between about 240 C. and 306 C.

3. The process of claim 1 wherein the polymeric composition is aterpolymer of acrylonitrile, methyl acrylate and a styrene sulfonatesalt.

4. The process of claim 1 wherein fibers having a stick temperaturebetween about 240 C. and 260 C. are heated from a temperature of about220 C. to a temperature of about 275 C. at a rate of about 1 C. perminute and maintained at about 275 C. for at least 5 minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,799,915 Barnett July 23, 1957 2,913,802 Barnett Nov. 24, 1959 OTHERREFERENCES Houtz: Textile Research Journal, November 1950, pp. 797 and798.

l 4 i l l i l l i

1. IN A PROCESS FOR PROVIDING FIREPROOF PROPERTIES BY HEATING FIBERSSPUN FROM A LINEAR POLYMERIC COMPOSITION ESSENTIALLY CONSISTING OFBETWEEN 85% AND 99% ACRYLONITRILE IN POLYMER FORM AND BETWEEN 15% AND 1%OF OTHER POLYMER COPOLYMERIZED WITH ACRYLONITRILE, THE POLYMERICCOMPOSITION CONTAINING NO HALOGENS ATTACHED TO ANY POLYMER CHAIN, THEFIBERS HAVING BEEN DRAWN TO AT LEAST 300% OF THEIR ORIGINAL AS-SPUNLENGTH; THE IMPROVEMENT, FOR PRODUCING FIREPROOF FIBERS WHICH ARE STRONGAND FLEXIBLE ENOUGH TO BE SPUN INTO TEXTILE YARNS, OF HEATING SAIDFIBERS IN THE FORM OF A FIBROUS MASS WITH A MAXIMUM BULK DENSITY OF 0.2GRAM PER CUBIC CENTIMETER, HAVING A FIBER STICK TEMPERATURE OF AT LEAST180*C., FROM A TEMPERATURE LOWER THAN 20*C. BELOW THE STICK TEMPERATUREOF THE FIBERS AT A RATE OF LESS THAN 3*C. PER MINUTE TO A TEMPERATUREBETWEEN ABOUT 260*C. AND ABOUT 500*C., IN AN OXIDIZING GASEOUS MEDIUMCONTAINING OXYGEN UNTIL THE FIBERS BECOME FIREPROOF.